In many instances, an offshore vessel connects with a riser extending from an underwater installation, such as a subsea well or manifold at the sea floor. The vessels move up and down and horizontally relative to the sea floor with the oscillations of the waves, currents, tides, winds and other environmental loading. The mechanism supporting the riser at the vessel maintains relatively constant tension on the riser during these oscillating movements. Riser tensioners, such as ram type tensioners (which push up on the riser from below) and hanging type tensioners (which pull up on the riser from above), may be used to support the riser tension and counteract movement of the vessel.